Power ballasts are known for providing regulated power to gas discharge tubes or lamps for the purpose of delivering a high voltage for ionizing the gas in the gas discharge tubes, to effect current flow therethrough and the emission of visible light therefrom. Such power ballasts also must provide the required voltage and current to ensure proper operation of the lamps subsequent to initial ionization.
Known solid state power supplies or power ballasts, such as ones described in U.S. Pat. No. 5,097,182 ("the '182 patent"), typically include a power transformer. Current flow through a primary winding of the power transformer is controlled by a power transistor which forms a major component of oscillator circuitry used to drive the load. A voltage sensing circuit is connected to the power transistor to limit the current flowing in the primary winding of the transformer, and is designed to limit current to prevent saturation of the magnetic core of the power transformer.
In the ballasts implemented in the prior art, the input or line voltage is rectified and supplied to a large input filter capacitor (e.g. C1 in the '182 patent). The input filter capacitor delivers energy to the circuit during each cycle when the line voltage falls below its peak value. Disadvantageously, such an input capacitor seriously degrades the power factor of the power supply or ballast. The effect of such a capacitor, as illustrated by FIGS. 1A and 1B herein, is that input current (FIG. 1B) lags the input voltage by a significant amount (in some cases the lag can be as much as 60 degrees). Furthermore, the input current is delivered in the form of input current spikes. It is generally the case that the larger the input capacitor used, the greater the current lag with respect to the input voltage, and the higher the amplitude of the current spikes. Such an input filter capacitor also disadvantageously results in an input surge upon powerup while the capacitor charges.
Power factor correction circuitry is known to be implemented to substantially overcome the degradation of a capacitively filtered A.C. input. U.S. Pat. No. 5,146,398 ("the '398 patent"), discloses circuitry for "active" power factor correction. The power factor correction circuitry used in the '398 patent to improve the ratio of real power to apparent power, includes a frequency and amplitude modulated boost converter that forces the input current to have virtually the same shape as the input voltage. Such active power factor correction as known in the prior art requires the addition of a conventional boost converter circuit that includes a large input choke (L1 in the '398 patent). Disadvantageously, such circuitry including the addition of the input choke, is bulky, takes up significant amounts of space, and is relatively expensive.
Additional circuitry is typically incorporated in known power supplies or power ballasts to effect short circuit, overload or overcurrent protection. Elaborate "failsafe" circuitry, such as described in U.S. Pat. No. 5,075,598, may be implemented and require the addition of sensing circuitry to sense for any unusual increase in current between the power transformer and ground. Disabling circuitry will also be required in a feedback path to the power transistor to disable oscillation and high voltage output. The additional sensing and disabling circuitry can add significant cost to the power supply or ballast. Furthermore, the additional circuitry takes up space which will disadvantageously increase the size and weight of the supply.